Digital copier magnification and distortion correction

ABSTRACT

A method of and an apparatus for reducing or eliminating scan line magnification errors in digital copiers that use an input scanner and a raster output scanner. The input scanner integration period, and thus the input scan line rate, is synchronized with the rotation of the raster output scanner, and thus the output scan line rate. Synchronization might be achieved using either a one to one input scan line to output scan line ratio, or by scanning multiple output scan lines for each input scan line. Beneficially, the principles of the present invention are implemented by synchronizing the input scan rate of an input scanner with the rotation of a polygon of a raster output scanner using a start of scan detector.

This invention relates to digital copiers. In particular, this inventionrelates to reducing defects caused by polygon motion variations indigital copiers having an input scanner and a raster output scanner.

BACKGROUND OF THE INVENTION

Electrophotographic marking is a commonly used method of copying orprinting documents. Electrophotographic marking is performed by exposinga substantially uniformly charged photoreceptor with a light imagerepresentation of a desired document. In response, the photoreceptordischarges, creating an electrostatic latent image of the desireddocument on the photoreceptor's surface. Toner particles are thendeposited onto that latent image, forming a toner image. That tonerimage is then transferred from the photoreceptor onto a substrate suchas a sheet of paper. The transferred toner image is then fused to thesubstrate, usually using heat and/or pressure, thereby creating a copyof the desired image. The surface of the photoreceptor is then cleanedand recharged for the production of another image.

One way of exposing the photoreceptor is to use a Raster Output Scanner(ROS). A ROS is comprised of a laser light source (or sources) and arotating polygon having a plurality of mirrored facets. The light sourceradiates a laser beam onto the polygon facets. That beam reflects fromthe facets and strikes the photoreceptor, producing a light spot. As thepolygon rotates the spot traces lines, referred to as scan lines, on thephotoreceptor. By moving the photoreceptor as the polygon rotates, thephotoreceptor is raster scanned by the spot. During scanning, the laserbeam is modulated according to image information so to produce thedesired latent image.

The foregoing broadly describes the operation of an electrophotographicmarking machine. Such marking machines are often used in digitalcopiers. In addition to an electrophotographic marking machine, digitalcopiers include an input scanner that scans an image on an originaldocument so as to produce a digital representation of that image. Then,the electrophotographic marking machine reproduces that image, possiblyafter being modified according to one or more user requirements.

Digital input scanners typically employ one or more arrays ofphotosensitive elements, such as charge coupled devices (“CCDs”), anillumination lamp, and an optical system. The lamp illuminates anoriginal document and the optical system focuses light reflected fromthe illuminated document onto the photosensitive elements. Since thereflected light intensity varies according to the image on the document,the photosensitive elements are able to convert the reflected light intoelectrical signals that represent the document. Typically, the documentis scanned as a sequence of line images, with each line image beingreferred to as a scan line. Thus, two uses are made of the term—scanline—, one referring to lines that are produced by sweeping the laserbeam spot across the photoreceptor, the other referring to a line imagescanned by the input scanner.

Conventionally, each input scan line is imaged during an integrationperiod of a predetermined duration. During the integration period thephotosensitive elements produce charges that are proportionate with theintensity of their input light. Those charges are accumulated on acapacitor. At the end of the integration period, the stored chargesresult in a potential that is then digitized to represent the chargebuildup. The duration of the integration period, which must besufficiently long to fully integrate the image line being scanned, yetnot so long as to allow the photosensitive elements to saturate, is setby a fixed rate clock signal in the prior art.

It can be seen that input scanners and raster output scanners are bothcharacterized by scan lines. An input scanner might scan 400 lines perinch while a raster output scanner might write 1200 lines per inch. Inthe prior art, the input scanner scan rate is set by an oscillator thatis derived from a crystal source (possibly via a microprocessor timerthat is programmed to slightly vary the oscillator frequency). However,because the output scan rate depends upon the mechanical rotation of apolygon, the output scan rate is difficult to control accurately. In theprior art the resulting variations between a highly stable input scanrate and a far less stable output scan rate typically did not matterbecause the digitized input image is usually stored in a “buffermemory.” The input image data is then read from the buffer memory asrequired. Essentially, in the prior art the input scanner data flow issynchronized with polygon motion.

While the prior art scheme of handling polygon motion variations issuccessful, it is not suitable for highly cost sensitive digitalcopiers. Not only must a buffer memory and an input scanner oscillatorbe provided, but also synchronization circuitry between the buffermemory and the polygon is required. Otherwise, variations in polygonrotational velocity (which might be +/−0.1% of nominal) would result in“slow scan magnification errors.” The slow scan direction is thedirection perpendicular to the scan line traced by the spot and isbrought about by the motion of the photoreceptor. Basically, if thepolygon is rotating too slow the photoreceptor motion would cause theimage to magnified in the slow scan direction, if the polygon isrotating too fast, the photoreceptor motion would cause the image to bereduced in the slow scan direction.

Because scan line magnification errors are detrimental to print qualitya technique of reducing or eliminating such errors would be beneficial.Even more beneficial would be a technique of reducing or eliminatingscan line magnification errors that is suitable for cost sensitivemachines.

SUMMARY OF THE INVENTION

This invention relates to a technique of reducing or eliminating scanline magnification errors. According to the principles of the presentinvention input scanner integration period is synchronized with therotation of the raster output scanner. Synchronization might be achievedeither directly, or by using an integral multiple of the input scanrate. By synchronizing the input scan rate with the output scan rate,slow scan multiplication errors are reduced or eliminated.

Beneficially, principles of the present invention are implemented bysynchronizing the input scan rate of an input scanner with the rotationof a polygon of a raster output scanner using a start of scan detector.

BRIEF DESCRIPTION OF THE DRAWINGS

The principles of the present invention will become apparent from thedescriptions that follow when read in conjunction with the accompanyingfigures, wherein:

FIG. 1 is a schematic view depicting an illustrative electrophotographicprinting machine that incorporates the present invention; and

FIG. 2 is a simplified schematic view of selected machine elements ofthe electrophotographic printing machine shown in FIG. 1.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

FIG. 1 shows an exemplary digital copier 3 that is in accord with theprinciples of the present invention. Generally, the printing systemincludes an input scanner 4, a controller section 6, and a printer 8.The input scanner 4 includes a transparent platen 20 on which a documentbeing scanned is located. One or more photosensitive element arrays 22,beneficially charge couple devices (CCD) and a lamp 23 are supported forrelative scanning movement below the platen 20. The lamp illuminates thedocument on the platen while the photosensitive element array 22produces image pixel signals from the light reflected from the document.After suitable processing the image pixel signals are converted todigital data signals that are sent to the controller section 6.

The control section 6, sometimes called an electronic subsystem (ESS),processes the digital data signals from the scanner section so as toenable reproduction of the image on the document. The control section 6includes control electronics that prepares and manages the flow ofdigital data from the input scanner 4 to the printer 8. The controlsection 6 may include a user interface suitable for enabling an operatorto program a particular print job, a memory for storing information,and, specifically important to the present invention, circuitry forsynchronizing and controlling the overall operation of the copier 3. Thecontroller section 6 sends the processed digital data signals to theprinter 8 as video data signals.

The printer 8 is a laser-printer that includes a raster output scannerthat produces a latent electrostatic image on a photoreceptor 40. Theraster output scanner includes a laser diode 30 that produces a laserbeam 32 that is modulated in accordance with the video data signals fromthe control section 6. The video data signals encode the laser beam withinformation suitable to reproduce the scanned in image. The laser beam32 is directed onto a rotating polygon 34 that has a plurality ofmirrored facets 36. The polygon is rotated by a polygon motor 38. As thepolygon rotates, the laser beam 32 reflects from the facets and sweepsacross the photoreceptor 40 as the photoreceptor moves in the direction41. The sweeping laser beam exposes an output scan line on thephotoreceptor 40, thereby creating an output scan line latentelectrostatic image. As the photoreceptor moves in the direction 41, byproperly modulating the laser diode 30 subsequent output scan linesproduce an electrostatic latent image on the photoreceptor thatrepresents the desired image.

Before exposure, the photoreceptor is charged by a corotron 42. Afterexposure, the electrostatic latent image is developed by a developer 44.The result is a toner image on the photoreceptor 40. That toner image istransferred at a transfer station 46 onto a substrate 60 that is movedfrom an input tray 62 to the transfer station by a document handler 58.After transfer, the substrate is advanced by a document transport 49into a fusing station 50. The fusing station permanently fuses the tonerimage to the substrate 60. After the toner image is transferred acleaning station 45 removes residual toner particles and other debris onthe photoreceptor 40.

After fusing, the substrate 60 is passed through a decurler 52. Thedecurler 52 bends the sheet in a first direction to put a known curl onthe substrate, and then it bends the substrate in the opposite directionto remove that curl. Forwarding rollers 53 then advance the substrateeither to an output tray 68 (if simplex printing or after the fusing ofa second image in duplex operation) or to a duplex inverter 56 thatinverts the substrate. An inverted substrate travels via a transport 57back into the document handler 58 for registration with a second tonerimage on the photoreceptor 40. After registration, the second tonerimage is transferred to the substrate at transfer station 46. Thesubstrate then passes once again through the fuser 50 and the decurler52. The forwarding rollers 53 then advance the substrate to the outputtray 68.

The foregoing generally describes the general operation of the digitalcopier 3. The principles of the present invention directly relate tosynchronizing the operation of the input scanner 4 with the rotation ofthe polygon 34. To that end, the digital copier 3 includes astart-of-scan detector 80 that senses when the laser beam is at apredetermined position. The output of the start-of-scan-detector 80 isinput to the controller section 6.

FIG. 2 assists the explanation of a preferred embodiment of the presentinvention. A start-of-scan-detector 80 having a sensor 82, beneficiallya photodiode, senses when the laser beam 32 is at a predeterminedposition. The output of the sensor 82 is applied to a pulse generator 84that generates a start-of-scan signal. The start-of-scan signal isapplied to a synchronizer 90 within the control section 6.

Also applied to the control section 6 via a bus 92 are the digital datasignals from the photosensitive array 22. In particular, the digitaldata signals are the outputs of an integrator 94 within thephotosensitive array 22. That integrator receives a plurality ofcharging currents, one charging current from each photosensitive elementof a CCD array 96. Those charging currents are proportional to thenumber of photons that strike the photosensitive elements. In turn, thenumber of photons are dependent upon the image on the document on theplaten 20 (reference FIG. 1). The integrator also receives an integratesignal from the synchronizer 90 via a line 95. In some cases thatintegrate signal might just be the start of scan signal In other casesthe integrate signal might occur a predetermined period of time after astart of scan signal. In other case the integrate signal might onlyoccur once for every N number of start of scan signals (where N isbeneficially an integer). However, when an integrate signal is received,the integrator begins accumulating charging currents from thephotosensitive elements. When the next integrate signal is received theintegrator compares the accumulated charges against a predeterminedcharge level. If the accumulated charge from a particular photosensitiveelement exceeds the predetermined charge level the integrator outputs alogic HIGH on a bus 92 line that corresponds to that photosensitiveelement. Otherwise, the integrator outputs a logic LOW on the bus 92line for that photosensitive element. Therefore, the outputs of thephotosensitive array 22 are synchronized in some fashion withstart-of-scan signals, and thus with the rotation of the polygon 34.

It should be apparent that the input scanner and the output scanner donot need to have the same number of scan lines. If the output scan linerate (in scan lines per centimeter) is an integer multiple (N) of theinput scan line rate, by producing one integrate signal for every Nstart-of-scan signals the input scanner can be synchronized with theoutput scanner. It is also possible to start integration based upon theoccurrence of an integrate signal, but to stop integration a fixedperiod of time later.

Still referring to FIG. 2, the digital data from the photosensitivearray 22 is stored within the controller section 6 in a shift register98. The operation of the shift register is synchronized with theoperation of the integrator via the integrate signal on the line 95 suchthat the signals from the integrator are “captured” by the shiftregister. The information stored in the shift register is then clockedout of the shift register to a laser driver 100 according to a pixelclock 102 within the control section. The laser driver then drives thelaser 30 according to the digital information.

Still referring to FIG. 2, while the operation of the input scanner issynchronized to the rotation of the polygon 34, it is still beneficialto maintain stabilize the angular frequency of the polygon 34. To thatend, the motor 38 that turns the polygon 34 is controlled by a motorspeed control 35. Additionally, it is often beneficial to use a mirror97 to reflect light from the sweeping laser beam 32 into the start ofscan sensor 80. This enables the start of scan sensor to be placed in amore convenient location.

It is to be understood that while the figures and the above descriptionillustrate the principles of the present invention, they are exemplaryonly. Others who are skilled in the applicable arts will recognizenumerous modifications and adaptations of the illustrated embodimentsthat will remain within the principles of the present invention.Therefore, the present invention is to be limited only by the appendedclaims.

What is claimed:
 1. An digital copier, comprising: a photoreceptormoving in a process direction; a raster output scanner including a lasersource that produces a laser beam and a rotating, multifaceted polygonfor reflecting said laser beam as a sweeping beam across saidphotoreceptor so as to produce a plurality of output scan lines; a startof scan detector for detecting said sweeping beam and for producing aplurality of start of scan signals, wherein the time between start ofscan signals depends upon the rotation of said multifaceted polygon; andan input scanner for digitizing an image on an original source, saidinput scanner including a light source for radiating light onto theoriginal source; an array of photosensitive elements for producing aplurality of charging currents, wherein each current depends upon thelight reflected from a pixel area of the image on the original source;and an integrator for integrating said plurality of charging currentsfor an integration period; wherein said integration period is a functionof the time between start of scan signals.
 2. A digital copier accordingto claim 1, wherein said integration period is a an integer multiple ofthe time between start of scan signals.
 3. A digital copier according toclaim 2, wherein said integer multiple is one.
 4. An digital copier,comprising: a photoreceptor moving in a process direction; a rasteroutput scanner including a laser source that produces a laser beam and arotating, multifaceted polygon for reflecting said laser beam as asweeping beam across said photoreceptor so as to produce a plurality ofoutput scan lines; a start of scan detector for detecting said sweepingbeam and for producing a plurality of start of scan signals, wherein thetime between start of scan signals depends upon the rotation of saidmultifaceted polygon; and an input scanner for digitizing an image on anoriginal source, said input scanner including a light source forradiating light onto the original source; an array of photosensitiveelements for producing a plurality of charging currents, wherein eachcurrent depends upon the light reflected from a pixel area of the imageon the original source; and an integrator for integrating said pluralityof charging currents for an integration period; wherein said integrationperiod is controlled by start of scan signals.
 5. A digital copieraccording to claim 4, wherein said integration period is a an integermultiple of the time between start of scan signals.
 6. A digital copieraccording to claim 5, wherein said integer multiple is one.
 7. Andigital copier, comprising: an input scanner for digitizing an image onan original document, said input scanner including a platen for holdingthe original document; a light source for radiating light onto theoriginal document; an array of photosensitive elements for producing aplurality of charging currents, wherein each charging current dependsupon the light reflected from a pixel area of a line of an image on theoriginal document; a scanning device producing relative motion betweensaid original document and said elongated array of photosensitiveelements such that said elongated array of photosensitive elementssequentially scans said image; and an integrator for integrating saidplurality of charging currents for integration periods that arecontrolled by integration pulses, said integrator for producing aplurality of digital signals that represent said line of said image atthe end of each integration period; a printer for printing an image on asubstrate, said printer including a photoreceptor moving in a processdirection; a raster output scanner including a laser source thatproduces a laser beam that is modulated with image information, saidraster output scanner further including a rotating, multifaceted polygonfor sweeping said modulated laser beam across said photoreceptor so asto produce a plurality of output scan lines, said printer furtherincluding a start of scan detector for detecting said sweeping modulatedlaser beam and for producing a plurality of start of scan signals,wherein the time between successive start of scan signals depends uponthe rotation of said multifaceted polygon; and a controller section forreceiving said plurality of digital signals that represent theintegrated charges and for using those digital signals to produce imageinformation that modulates said laser beam, said controller further forreceiving said start of scan signals and for using said start of scansignals to produce said integration pulses; wherein said controllerproduces integration pulses at a rate that depends upon the rate of saidstart of scan signals.
 8. A digital copier according to claim 7, whereinsaid start of scan signals occur at a rate that is a fixed multiple ofthe rate of said integration pulses.
 9. A digital copier according toclaim 7, wherein said fixed multiple is an integer multiple.
 10. Adigital copier according to claim 9, wherein said integer multiple isone.